To install the lower post, dig a post hole at least 18 inches into the ground. Set a 4x4 post into the hole, and fill it with concrete to secure the post. While the concrete is wet, use a 4-foot carpenter’s level to ensure the post is vertical in all directions.

Use a tape measure to mark a line 35 inches from the top of the leading edge of the bottom step. Use a speed square to mark a horizontal line across the front of the post at the mark. Lay a board along the steps, and measure the angle between the board and the post. Transfer the angle to the post to make a cutting line that intersects the 35-inch mark on both sides of the upright.

Use a circular saw to cut the top of the post along the angle line. Since the blade does not cut all the way through the post, cut from both sides. Make a similar cut for the railing post at the top of the deck.

Install the top rail between the posts, extending several inches below the bottom post. Cut the rail to the desired length, and then sand and finish it.